Of ATP. Thus, in terms of ATP production, FHDH is not as energy rich as NADH.
In addition to the production of NADH and FADH, the Krebs cycle results in direct formation of an energy-rich compound guanosine triphosphate (GTP) (see bottom of Fig. 3.18 ). GTP is a high-energy compound that can transfer is terminal phosphate group to ADP to form ATP. The direct formation of GTP in the Krebs cycle is called substrate-level phosphorylation. lt accounts for only a small amount of the total energy conversion in the Krebs cycle because most of the Krebs cycle energy yield (i.e., NADH and FADH) is taken to the electron transport chain to form ATP.
Up to this point we have focused on the that carbohydrates play in producing acetyl-COA to enter the Krebs cycle. How do fats and proteins undergo aerobic metabolism the answer can be found in Fig. 3.19. Note that fats (triglycerides) are broken down to form fatty acids and glycerol. These fatty acids can then ndergo a series of reactions to form acetyl-CoA (called beta oxidation; see A Closer Look 3.4 for details) and thus enter the Krebs cycle (8). Although glycerol can be converted into an inter-mediate of glycolysis in the liver, this does not occur to a great extent in human skeletal muscle fuel source during exercise (16).As mentioned previously, protein is not considered a major Fuel source during exercise because it contributes only 2% to15% of the